There is a class of powered mobile machines, such as floor scrubbers and sweepers, in which the machine is operated by an attendant walking behind it. Machines of this type are used to clean large floor areas such as factories, and for such indoor service a battery powered machine is usually preferred. In the case of a scrubber, the large areas to be scrubbed necessitate sizeable water tanks on the machine which, along with the heavy batteries, cause the machine to be so heavy that a powered traction drive is essential and power assisted steering is desirable. Some walk-behind scrubbers use one traction motor driving two drive wheels through a differential gear. On such scrubbers individual clutches or brakes on the drive wheels have been used to assist steering, but some of these have been less durable than desired. Other scrubbers have been fitted with two traction motors, one to power each drive wheel, with means being provided for the operator to control the speeds of these motor in unison for speed control and differentially for power steering. This is an improvement over the single motor approach, and is coming to be the common method, at least in the heavier machines. However, the control means which have been used still do not provide optimum ease of handling. They tend to respond slower than smaller scrubbers which are light enough to be steered manually. A faster steering response can be obtained if the operator can help steer the machine by manually pushing on the controls to supplement the power steering, but pushing on the controls of some models may increase the machine travel speed to maximum before any manual steering occurs, which could be hazardous. Some machines provide a programmed rate of change in the differential speed of the two motors, which turns the machine at a pre-established rate even though the operator might want to make a quicker turn under some circumstances.
Therefore in this class of machines there is a need for a control system that will make a machine more responsive and maneuverable than previously available models. It should provide full power steering while at the same time permitting the operator to apply manual force to the machine to assist the steering without affecting the speed control or power steering. It should cause steering and travel speed to respond in proportion to the degree of movement given to the controls rather than at a preprogrammed rate. Also, it should be capable of making a turn as sharp as turning about the centerline of the machine, and without forward or reverse travel if desired, for maximum maneuverability in tight spaces. The present invention provides these improvements.